The present invention relates to implantable fusion devices and methods for their use. More particularly, the present invention relates to interbody fusion devices formed of bone that may be utilized in spinal fusions.
A variety of interbody fusion implants are available for spinal fusion procedures. These implants have been manufactured of various materials including steel, titanium, composites, allograft, xenograft or other biocompatible materials. These implants may be inserted using fixed protective tubes to protect surrounding neurological and vascular structures or through an unprotected open procedure. One limitation on the size of a device inserted into the disc space is the size of the opening through surrounding tissue that is available to gain access to the disc space. From a posterior approach to the spine, the dura and nerve roots must be mobilized to gain access to the disc space. Similarly, from an anterior approach, the aorta and vena cava must be mobilized to gain access to the disc space. Such mobilization is often limited by the anatomical structures, thus resulting in a relatively small access site in comparison to the size of the disc space. Removal of additional ligaments and bone to enlarge an entrance to the disc space may de-stabilize and weaken the joint between two adjacent vertebra. Moreover, excessive retraction of vessels and neural structures to create a large access opening may result in damage to these tissues. Thus, prior procedures have been limited to placing a first device passable through the available opening on one side of the spine and mobilizing the tissue or vessels to place another similar implant on the opposite side of the spine. Each implant being limited in size by the available access site.
In response, expandable implants have been developed from biocompatible materials such as titanium and composites. These devices rely on hinges or selective deformation of the implant material to permit expansion after they are positioned in the disc space. While such devices have a reduced insertion configuration and an expanded spacing configuration, the materials utilized to form the implants are synthetic and will not incorporate into adjacent bony tissues. While bone offers much improved incorporation, the inherent brittle nature of bone resulting from a high mineral content, particularly load-bearing cortical bone, severely limits its potential deformation. Typically, for example, cortical bone consists of approximately 70% mineral content and 30% non-mineral matter. Of this non-mineral matter, approximately 95% is type I collagen, with the balance being cellular matter and non-collagenous proteins.
Bone grafts, in conjunction with other load-bearing implants, have commonly been used in a fixed shape, pulverized, or as pliable demineralized bone. One form of a pliable bone graft is a demineralized bone material typically in the form of a sponge or putty having very little structural integrity. While a demineralized bone segment may retain properties suitable to support bone ingrowth, the structural properties of the bone are altered by removal of its mineral content. Thus, such bone sponges and putties may not typically be used in load-bearing applications.
Therefore, there remains a need for a strong bone implant having an area of flexibility.